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Be scope aware

In JavaScript, scope is everything. A scope can be thought of as the space where certain variables exist.

The default (or global) scope is the

window

in JavaScript. Variables created in the window scope aren’t

destroyed until the Web page is unloaded from the browser. Each function you define is another scope

under that global scope. All variables created within the function exist only within the function scope

and are destroyed when execution leaves the function.

You can think of scopes in JavaScript as a hierarchical tree. When a variable is referenced, the JavaScript

interpreter looks to the most recent scope to see if it exists there. If not, it goes up to the next scope, and

the next, and so on, until it reaches the

window

scope. If the variable is not found in the

window

scope,

you receive an error during execution.

Every time the interpreter goes up to another scope in search of a variable, execution speed suffers.

Variables that are local to a scope lead to faster script execution when compared to global variables. The

less distance the interpreter has to travel up the tree, the faster your script runs. But what exactly does

this mean? Consider the following example:

var sMyFirstName = “Nicholas”;

function fn1() {

alert(sMyFirstName);

}

function fn2() {

var sMyLastName = “Zakas”;

fn1();

}

function fn3() {

var sMyMiddleInitial = “C”;

fn2();

}

fn3();

When

fn3()

is called on the last line, a scope tree is created (see Figure 19-1).

The importance of understanding scopes becomes evident in this example as you travel down the scope

tree. The function

fn3()

calls

fn2()

, which calls

fn1()

. The function

fn1()

then accesses the

window

level variable

sMyFirstName

, but in order to locate this variable, the interpreter has to look back up the

scope tree all the way to the window scope. This takes significant time. Finding ways to take advantage

of JavaScript scoping is vital in optimizing execution time. You can do a few easy things to help the

interpreter locate variables faster.

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